1. Field of the Invention
The present invention relates to a mobile communication terminal apparatus having a function of requiring, for example, handoff of a base station, a control circuit of the apparatus, and a handoff control method in a cellular mobile communication system.
2. Description of the Related Art
In recent years, cellular mobile communication systems have rapidly spread. In the cellular mobile communication system, a plurality of base stations are scattered/located in a service area and these base stations form respective radio zones called cells. Moreover, the mobile communication terminal apparatus is connected to a base station of the radio zone in which the terminal apparatus exists over a radio channel, and the base station is connected to the terminal apparatus of a communication destination via a communication network. This realizes communication among the terminal apparatuses.
Additionally, in this type of system, a message is exchanged between the mobile communication terminal apparatus and the base station via the radio channel during waiting or communicating, so that desired control is executed. In this case, a request message which needs to be securely transmitted to the communication destination requires acknowledgment. To realize the acknowledgment, the base station having received the request message adds a specified number included in the received request message to a response message of acknowledgment, and returns the response message to the mobile communication terminal apparatus as a transmitter. The mobile communication terminal apparatus having transmitted the request message retransmits the request message at a constant time interval, until the response message of acknowledgment can be received. On receiving the response message of acknowledgment, a procedure of retransmitting the request message is ended.
Moreover, the mobile communication terminal apparatus can transmit a plurality of request messages in the same period, and wait for a response of acknowledgment. However, an upper-limit value of the number of request messages which can be transmitted in the same period is determined. Therefore, when the number of transmitted request messages reaches the upper-limit value, and a new request message requiring the acknowledgment response is generated, the new request message is set to a transmitting waiting state until reception of the acknowledgment response to any one of the plurality of transmitted request messages.
The aforementioned message transmission control procedure will be described by means of an example for executing handoff in a system in which a code division multiple access (CDMA) method is employed.
The CDMA mobile communication terminal apparatus measures a reception level of a pilot signal transmitted from the base station which is establishing synchronization during waiting or communicating. Moreover, the respective reception levels of the pilot signals transmitted from a plurality of base stations (neighboring base stations) located around the base station which is establishing the synchronization are also measured. A strength of reception-level is represented by a ratio (Ec/Io) of a whole reception energy (Io) of the pilot signal after inverse diffusion to an energy (Ec) of the pilot signal arriving from the base station as a measurement object. Moreover, when the reception level of the pilot signal arriving from the peripheral base station has a strength of a predetermined level or more strength, the following determinations are performed. In one determination, it is determined whether or not a strength (Ec/Io)PNa2 of the reception level of a pilot signal PNa2 arriving from the base station establishing the synchronization is less than a value designated by the base station. In the other determination, it is determined whether or not a strength (Ec/Io)other of the reception level of each pilot signal arriving from another base station is more than another value designated by the base station. Moreover, when answers to these determinations are both affirmative, a handoff request message called a pilot strength measurement message (PSMM) is generated and transmitted to the base station establishing the synchronization. In this handoff request message, the reception level of the pilot signal arriving from the base station which is establishing the synchronization, and the reception levels of the respective pilot signals arriving from the respective neighboring base stations are inserted.
On the other hand, on receiving the handoff request message from the mobile communication terminal apparatus, the base station determines a base station as a handoff destination based on the reception level inserted in the message. Moreover, a notice of a name of the base station as the determined handoff destination is transmitted to the mobile communication terminal apparatus which requires handoff. On receiving the notice, the mobile communication terminal apparatus first returns the response message indicating the acknowledgment to the base station. Subsequently, the base station as a synchronization destination is switched in response to the notices. A handoff processing of the mobile communication terminal apparatus is performed in this manner.
When a plurality of handoff requests are generated, the mobile communication terminal apparatus generates handoff request messages for the respective handoff requests, and use separate sequence numbers (SEQNo) to transmit these messages to the base station. Moreover, the apparatus waits for the acknowledgment response to each handoff request message from the base station. Subsequently, with respect to the message to which the response of acknowledgment is not returned in a fixed time, transmission of the handoff request message is repeated according to a predetermined access sequence by a designated number of times (e.g., nine times at maximum). FIG. 12 shows one example of the sequence. In FIG. 12, (SEQ1(1)) indicates a first transmission with a sequence number 1, and (SEQ1(2)) indicates a second transmission with the sequence number 1. Other sequence numbers are similarly displayed.
Moreover, the mobile communication terminal apparatus can transmit a plurality of handoff request messages in the same period. However, an upper limit of the number of messages which can be transmitted in the same period is defined. Therefore, when the number of handoff request messages being transmitted reaches the upper-limit value, and another new handoff request message is generated, the new handoff request message is set to the transmission standby state until reception of the acknowledgment response to any one of the handoff request messages being transmitted.
Additionally, in the conventional message retransmission procedure, each message is repeatedly retransmitted while a message content of start of transmission is held as it is. Therefore, when the number of retransmissions increases, the message content having reached the base station already becomes old in some cases. Particularly, when the mobile communication terminal apparatus moves at a high speed, a position of the mobile communication terminal apparatus of a time at which the reception level from each base station is measured, and a position of the mobile communication terminal apparatus of a time at which the handoff request message actually reaches the base station largely change. Therefore, a difference between the notified measured value and the actual measured value increases. In this case, the base station determines the handoff destination in accordance with the old measured value which is different from the actual measured value. Therefore, there is a possibility that appropriate handoff is not performed.
Furthermore, when the number of messages waiting for the acknowledgment response reaches the upper-limit value, subsequently generated messages are set to the transmission standby state. Therefore, much time is taken from when the transmission standby state is canceled until the message is transmitted. Also in this case, the content of the message having reached the base station already becomes old in many cases, and as a result, handoff failure easily occurs.